Sergio Agüero's post-match confrontation with Villarreal players was sparked when the Manchester City striker broke into song in the tunnel.

Witnesses to the flare-up have reported that Agüero's singing incensed the Spaniards as they made their way off the pitch at the end of the Champions League tie on Tuesday, suspecting that his winning goal, four minutes into stoppage time,had been offside.

Stewards had intervened a few moments earlier after the Villarreal players confronted the match officials and, with tempers flaring, they took offence to a smiling Agüero singing an unspecified song in Spanish, close to the door leading to the away dressing room. In the following confrontation, several of Agüero's team-mates came to his defence. Coaching staff pulled them away before the trouble escalated.

Villarreal were described as the more aggressive of the two sets of players and the bad feeling continued in the form of strong condemnation from the club's vice‑president, José Manuel Llaneza, and the captain, Carlos Marchena.

Llaneza said: "What he [Agüero] has done has shown him up. It seems to me a lack of respect, ethics and comradeship. I thought he had learned something and grown up but I do not understand why he would taunt his fellow professionals about losing.

"I am not saying this because of the result. We have lost and that's that, and it has nothing to do with this. It is just because of what Agüero did. This is a player who should thank Spain for a lot, a great player who should not do things like this."

Agüero's reputation is not as a troublemaker and the Argentinian put a message on his Twitter account to offer his version of events.

"I have no interest in getting involved in controversies, but I have to deny having mocked a Villarreal player. Everyone can attest to my discipline and professionalism. I wouldn't do something like that."

However, the condemnation from Villarreal was strong enough to leave the sense that the return match in Spain on 2 November now has the potential for emotions to run high again. Marchena said: "You have to have class in football, especially in moments like that. What Agüero did was not acceptable and even more so because of the way he did it.

"These things are embarrassing and they tell you about the person who does them. You have to be a good person as well as a good sportsman and show class, which he hasn't got."

Agüero's goal entirely changed the complexion of Group A but an ultimately celebratory night could also lead to City being fined because of the way the blue and white sheets of paper that were held up before the game were screwed up and thrown on to the pitch. A Villarreal corner was held up at one point and Uefa has opened a disciplinary case, to be held on 17 November, that will probably result in a fine.

The conduct of the Villarreal team at the final whistle, when they angrily crowded around the referee and his assistants, may also lead to punishments. "We are waiting to receive the official match reports," a Uefa spokesman said. "We will look at them and decide if we need to act."

Agüero has now scored nine goals in eights starts and two substitute appearances since joining City from Atlético Madrid and the club seem relaxed about the post-match controversy, believing it to be sour grapes on the part of their opponents. Pablo Zabaleta was asked afterwards whether he would like Wayne Rooney or Agüero in his team for the Manchester derby on Sunday. "Easy for me – Sergio," he replied.

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